Phase Noise Compensation. Theory and Practice

Transcription

1 Phase Noise Compensation Theory and Practice

2 Trellis Demodulation Basics Legacy Single-Symbol Detection Basic Limiter-Discriminator operation Frequency in this bit above nominal è data = Frequency in this bit below nominal è data = Makes no use of adjacent symbols for error correction Trellis Detection Uses the phase tree for data detection Adjacent symbols are used to help decide on iffy bits Improves BER performance by 3.5 to 5. db What s a Phase Tree? Why is it useful? 2 Reinventing Telemetry

3 Ideal PCM/FM Phase Tree (h =.7) Phase in Cycles Time in bits 3 Reinventing Telemetry

4 Phase Trajectory Never Forgets If this bit is different, the phase trajectory is forever shifted Phase in Cycles Therefore, later bits can help make decisions about earlier bits Time in bits 4 Reinventing Telemetry

5 Trellis Demodulation Summary The basic premise of trellis demodulation is that the signal from the transmitter follows a known path through the phase tree. When the demod knows this, it can use a sequence of several symbols to help make better decisions about each individual bit. This process improves BER performance by about 3.5 to 5 db over conventional FM detection. However, this assumes that the transmitter is really following the "known", and "correct" phase tree, and this assumption is NOT always true. High phase noise can reduce the trellis detection gain. Why is that? 5 Reinventing Telemetry

6 Proprietary and Confidential to Quasonix Phase Noise Corrupts the Tree No phase noise: The entire trellis is helpful Time in bits Significant phase noise: A few bits of the trellis are helpful Time in bits Severe phase noise: Not a candidate for trellis demodulation Phase in Cycles Phase in Cycles Minimal phase noise: Several bits of the trellis are helpful Phase in Cycles Phase in Cycles Time in bits Time in bits Reinventing Telemetry

7 Phase Noise Impact Trellis demodulation is based on the assumption that the signal is following a predictable path through the trellis. If this is not true (due to high phase noise), then a trellis demodulator cannot provide the expected performance gain. Many legacy analog transmitters (a simple modulated VCO) have high phase noise. Vibration often further increases phase noise. Phase noise is generally more damaging at low bit rates. Phase Noise Compensation (PNC) gives back some of the trellis detection gain, by shortening the trellis observation span. When do I use PNC? 7 Reinventing Telemetry

8 When to Use PNC There is no bullet-proof test for whether PNC is needed, but there are good indicators. Turn on PNC if Demod is struggling to lock, even with good SNR Good SNR: Quality bar is above one-quarter height Eye pattern NEVER looks clean, like this Symptoms get worse when the transmitter is under vibration Symptoms get worse at low bit rates 8 Reinventing Telemetry

9 Know Your Transmitter If you know the brand and type of transmitter, these are good starting points Leave PNC off, if your TX is from Quasonix guaranteed Nova Engineering highly likely L3 probably, but digital transmitters only Turn PNC on, if your TX is from Microwave Innovations Emhiser Southern California Microwave L3 (analog transmitters) 9 Reinventing Telemetry

10 Future Developments Quasonix has pioneered the PNC feature, and continues to refine it. New algorithms are under development that can differentiate between phase noise and receiver front end noise. A fully automatic PNC mode will be available in Quasonix receivers when these algorithms are smart enough. This will be a free upgrade for most models, installed through reprogramming. Reinventing Telemetry

11 Questions Feel free to call Terry Hill Reinventing Telemetry

12 Thank You!